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The Impossibility of Approximate Agreement on a Larger Class of Graphs

Author Shihao Liu

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Author Details

Shihao Liu
  • Department of Computer Science, University of Toronto, Canada

Funding

This work is supported by the Natural Science and Engineering Research Council of Canada.

Acknowledgements

I want to thank my advisor Faith Ellen for her advice and many helpful discussions. I would also like to thank the anonymous reviewers for their helpful comments.

Cite AsGet BibTex

Shihao Liu. The Impossibility of Approximate Agreement on a Larger Class of Graphs. In 26th International Conference on Principles of Distributed Systems (OPODIS 2022). Leibniz International Proceedings in Informatics (LIPIcs), Volume 253, pp. 22:1-22:20, Schloss Dagstuhl – Leibniz-Zentrum für Informatik (2023)
https://doi.org/10.4230/LIPIcs.OPODIS.2022.22

Abstract

Approximate agreement is a variant of consensus in which processes receive input values from a domain and must output values in that domain that are sufficiently close to one another. We study the problem when the input domain is the vertex set of a connected graph. In asynchronous systems where processes communicate using shared registers, there are wait-free approximate agreement algorithms when the graph is a path or a tree, but not when the graph is a cycle of length at least 4. For many graphs, it is unknown whether a wait-free solution for approximate agreement exists. We introduce a set of impossibility conditions and prove that approximate agreement on graphs satisfying these conditions cannot be solved in a wait-free manner. In particular, the graphs of all triangulated d-dimensional spheres that are not cliques, satisfy these conditions. The vertices and edges of an octahedron is an example of such a graph. We also present a family of reductions from approximate agreement on one graph to another graph. This allows us to extend known impossibility results to even more graphs.

Subject Classification

ACM Subject Classification
  • Theory of computation → Distributed algorithms
  • Theory of computation → Computability
Keywords
  • Approximate agreement on graph
  • wait-free solvability
  • triangulated sphere

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